1. Field of the Invention
The present invention is directed to a method for the production of a light conductor structure which has an electrode arranged between the light conductors.
2. Prior Art
Light conducting structures, which have a pair of light conductors on a substrate with an electrode disposed therebetween, are known. Such structures are used as electrically controllable directional couplers in optical communication technology and act as on/off switches or cross-over switches. In addition such structures are used as electro-optical modulators.
A common feature of these structure is that the two light conductors possess a zone in which they are very closely adjacent to one another. Electrodes are arranged in this zone between the light conductors and besides each of light conductors. In this zone, a typical value for the spacing between the pair of light conductors is 3 .mu.m. This means that the electrodes must be precisely aligned in their position and that permissible tolerances in the location of the electrodes are less than 1 .mu.m.
A switch, which can be used as a modulator and which comprises two coupled light conductors which can be detuned relative to one another by applying an electrical field of the conductors, is disclosed in the article by H. F. Taylor, "Optical Switching and Modulation in Parallel Dielectric Waveguides," J. Appl. Phys., Vol. 44, No. 7, July 1973, pp. 3257-3262.
If the two light conductors are arranged so that they are closely adjacent over a sufficiently long path, they are optically coupled to one another. For example, in a coupling length L, a periodic change of optical energy takes place between the two conductors. If both light conductors are loss-free and their phases are matched to one another, the energy exchange is complete. If, however, the propagation constant for the light changes asymmetically in the light conductors, only a part of the energy is exchanged. Furthermore, there is also a change in the coupling length L over which the maximum exchange of energy will occur.
If the light conductors are produced from an electro-optical material, it is possible to control the index of refraction and thus the propagation constants in the light conductors by applying an electrical field to the light conductors. The coupling strength, which is dependent upon the distance between the intercoupled light conductors, upon the refractive indices of the light conductors, and the coupling length L, is selected to be such that when no electrical field is applied, the energy from one light conductor is completely coupled to the other light conductor. By applying an electrical field to the light conductors, the propagation constants of the two light conductors are detuned relative to one another in such a manner that for a given coupling length L, a part of the optical energy is first coupled into the other light conductor and then is coupled back again.
An example of a known type of light conductor structure or device is a modulator which is illustrated in FIG. 1 and generally indicated at 9. The modulator 9 includes a substrate 1, which consists of an electro-optical crystal, for example, lithium niobate (LiNbO.sub.3) and which has light conductors 2 and 3 which are arranged closely adjacent to one another along a coupling length L. The light conductors 2 and 3 are produced by diffusing a metal such as titanium or nickel into the selected areas of the substrate to increase the index of refraction of the substrate in the doped zones. An electrode 5 is located between the light conductors 2 and 3 and electrodes 4 and 6 are located beside each of the light conductors 3 and 2, respectively. By connecting the electrodes 4, 5 and 6 to voltage sources such as 7 and 8, it is possible to alter the optical properties of the light conductors 2 and 3.
However, when a central electrode 5 is arranged between the two light conductors 2 and 3, particular difficulties will occur. These difficulties are due to the fact that the distance between the coupled light conductors 2 and 3 is only between 1 .mu.m and 3 .mu.m and due to the fact that the central electrode 5 must be positioned as exactly as possible between the light conductors 2 and 3. Thus, the tolerances for the position of the electrode 5 are extremely small.